This invention relates to a capacity measuring device and is used in particular for weighing apparatus of the capacitive transducer type.
In such a weighing apparatus, the value of the load to be measured is transformed into that of a capacity. Where there is a zero load, the two electrodes of a capacitor are brought close to each other, thus defining a fairly high capacity value. When the load increases, the electrodes are moved away from each other and the value of the capacity decreases to a minimum value, which corresponds to the full scale deflection of the weighing apparatus.
In patent application No. 77 28 758 of Sept. 23, 1977, we proposed inserting measuring capacitors into a relaxation oscillator, the operating frequency of which is inversely proportional to the value of the capacity. With an arrangement of this type, a minimal frequency Fmin is thus obtained in the presence of zero load, and a maximum frequency Fmax is obtained in the presence of the maximum value of the load to be weighed. The arrangement is such that the frequency excursion which appears in the presence of an unknown load is very nearly proportional to the value of this load. The prior patent application also proposes using a standard capacity in addition to the capacity to be measured. The same oscillator sometimes functions with the measuring capacity and sometimes with the standard capacity.
Various difficulties result from this "multiplexing" of the same oscillator between the capacity to be measured and the standard capacity, in particular regarding adjustments required for controlling the general effect of the weighing apparatus.